Heading angle is a critical parameter for an aircraft. An Attitude and Heading Reference System (AHRS) typically calculates magnetic heading angle from measurements of earth's magnetic field taken from a magnetometer. The magnetometer is subject to measurement errors generated from the aircraft such as hard iron bias and soft iron, and measurement errors generated from the sensor such as scale factor and misalignment that corrupt the measurements and make it difficult to directly measure the earth's magnetic field. Hard iron bias refers to the effect of permanently magnetized material that introduces a bias to earth's magnetic field at the magnetometer location. Soft iron refers to material that is not permanently magnetized but distorts the earth's magnetic field at the magnetometer location. The effects of the magnetometer measurement errors can result in magnetic heading angle errors of 10 degrees or more.
Magnetic calibration is used to correct for the effects of magnetometer measurement errors so that magnetic heading angle can be determined accurately. Traditional compass rose magnetic calibration is done on the ground through the use of surveyed heading angles, which are compared to the AHRS magnetic heading angle. However, there are certain costs and risks that can be eliminated by replacing on-ground calibration with in-air magnetic calibration. These include the need for dedicated compass rose facilities; the time and cost for a tug vehicle and operator to rotate the aircraft; the cost of safety personnel around the aircraft when engines are running; the risk of an incorrect calibration due to magnetic interference from ground equipment; and the limitation of two dimensional magnetometer calibration.
In-air magnetic calibration addresses the foregoing problems and provides a full three dimensional magnetometer calibration with a brief one-time flight maneuver. However, in-air calibration still requires a true heading angle input. In one prior approach, the source of true heading angle for use in-air calibration was an inertial reference system (IRS). The need for an IRS on the aircraft made this approach limited, as the IRS may not be available in the aircraft architecture.